The invention described herein was made by an employee of the United States Government and may be manufactured and used by or for the Government for governmental purposes without payment of any royalties thereon or therefore.
1. Technical Field of the Invention
The present invention relates to wiring inspection techniques, and more specifically, it relates to techniques for identifying and determining the location of wiring insulation defects.
2. Description of the Prior Art
The presence of insulation defects in electrical wiring used in the Space Shuttle poses serious risks. Similar problems exist in all aircraft, as well as in industrial and other facilities. It is extremely difficult to identify cracked insulation even when it is exposed, and nearly impossible to detect it if it is not visually accessible.
Prior art technology for detecting insulation defects includes the use of Dielectric Withstand Voltage (DWV), the use of conductive media surrounding the wire, and the use of Time Domain Reflectometry (TDR).
DWV uses a relatively high electric potential to cause arcing to a surrounding component at ground potential. The discharge can then be detected to identify whether a particular conductor has a breach in the insulation. DWV does not have the ability to locate a defect in wire insulation, and if there is not a component at ground potential within approximately 0.010xe2x80x3 (xcx9c254 xcexcm) of the conductor, DWV used at normal voltages is not effective even for identifying the existence of an insulation defect. Higher voltages cannot be used for fear of harming undamaged insulation.
Because the arcing potential in atmosphere does not extend far enough to detect defects unless there is something at ground potential very near the insulation defect, a conductive medium (liquid or gas) can be used to enhance the conduction of the electricity from the wire to ground. Use of such a conductive medium enhances the capability of DWV to detect the existence of an insulation defect, but still does not locate it.
TDR uses a step or pulse of electricity, which reflects from changes in impedance in the wire, to measure the distance from that change. TDR is typically operated at low voltage (less than 25 volts, and usually less than 5 volts), and has essentially zero potential to arc in a normal atmosphere.
The use of TDR superposed on a direct current (DC) potential has been proposed as a means of increasing the distance to which TDR will arc, but even so, the arcing distance is limited to approximately 0.010xe2x80x3 (xcx9c254 xcexcm), since the DC potential will be limited to the same range noted for DWV, for the same reason. Further, a means of ensuring that arcing occurs on the TDR signal rather than on the DC current is required.
It is therefore desirable to provide a reliable technology for the detection and location of defects in the insulation of wire, and short and open circuits
It is an object of the present invention to provide a method and apparatus that enables remote detection and localization of short and open circuits and of very small defects in the insulation of wires, some of which heretofore required manual inspection and local access.
It is another object to combine a technique for avoiding premature arcing with Time Domain Reflectometry (TDR), Dielectric Withstand Voltage (DWV) and the use of conductive medium to remotely detect and locate wire insulation defects and short and open circuits.
These and other objects will be apparent to those skilled in the art based on the disclosure herein.
The invention remotely detects and locates very small breaches in wire insulation. The proposed innovation was inspired by the need to inspect shuttle wiring. Anticipated uses of the present invention include inspection of wiring in the space shuttle, in military and civilian aircraft and industrial plants. By combining several existing technologies (TDR, DWV and in some cases, the use of a conductive medium, none of which alone can achieve the stated goal) with an innovative concept for avoiding premature arcing, this new technique will overcome the limitations of each of the current individual technologies.
TDR makes use of the fact that a portion of a pulse or step function is reflected from a change in impedance in a conductor. The characteristic velocity of the conductor and the time for the reflection to return are used to calculate location.
DWV applies a predetermined voltage, usually DC, to a conductor, and notes if there is any arcing to a ground. It is limited by the fact that the usual voltage, about 1500V, is only enough to arc about 0.009xe2x80x3 (xcx9c229 xcexcm).
Incidents in the Shuttle Program and in the aviation community have resulted in a heightened interest in the possible aging of aerospace vehicle wiring and the potential consequences of this aging on safety.
Current technology does not permit remote detection and localization of small insulation defects, yet these defects can trigger catastrophic problems. For example, the wiring anomalies that triggered the incident on STS-93 very likely could not have been detected by any technique other than a visual or physical examination of the wire in question. The present invention would have allowed detection of breaches in the insulation of the same size as the one that caused this incident.
A significant development in the field, the invention achieves a goal that has been sought after by many organizations. The FAA, the Navy and the Air Force have all been funding research toward the goal of identifying and locating insulation defects. Thus far, none of the published research efforts known to the present inventor has resulted in a technique that is successful in non-destructively locating all insulation defects with breached insulation. The present development provides the ability to locate such defects. Various means may be used to provide such a ground if it is not otherwise available.